In conventional pressurized fluidized bed reactor power plants, a fluidized bed reactor is enclosed by a pressure vessel which is connected to a compressor for compressing gas (typically air) to superatmospheric pressure and delivering it to the pressure vessel at a pressure of about 2-100 bar. The compressed air is fed into a gas volume defined between the pressure vessel and the fluidized bed reactor. By combustion of coal or other carbonaceous fuel in the reactor, hot gases are generated. Particles are removed from the hot gases, and then they are fed to a turbine or a like expansion means, for example for generating electricity.
Disturbances may occur in the operation of the power plant, such as a sudden interruption in the fuel supply, loss of GT-generator load, a pulsation of the pressure in the gas volume, etc. Under these circumstances, it is necessary to stop the supply of pressurized air into the pressure vessel. In order to protect the turbine and compressor from damage caused by operational disturbances, it has been suggested in U.S. Pat. No. 4,744,212 (the disclosure of which is hereby incorporated by reference herein) to provide the conduit for supplying pressurized gas to the pressure vessel, and the conduit for transporting hot pressurized gas to the turbine, with valves which close these flow paths, thus protecting the compressor and turbine from damage by disconnecting them from the rest of the system. Also it is suggested to provide a short circuit conduit between the compressor and the turbine for passing gas from the compressor directly to the turbine when the valves are closed.
While the system described above can be effective under some circumstances, it is not entirely effective or practical. The shutoff valve in the conduit for the hot gases to the turbine must be manufactured to extremely tight tolerances, and if it is not it is incapable of completely closing off the hot gas discharge, and therefore it is inevitable that at least a portion of the gas may leak into the turbine. Also this valve is large and must endure extremely adverse thermal conditions, is expensive, and slow in operation, thereby handicapping the control of the entire system.
It also has been suggested in the prior art that the pressure in the pressure vessel be reduced by discharging the gas from the vessel into the atmosphere through valves in the outer wall of the vessel upon operational disturbances. However since the valve for sealing the hot gas conduit cannot be effectively completely closed, and if the supply of fuel is interrupted or completely stopped gas may enter the turbine through the reactor and filter. Since this gas is considerably cooler than the gases during normal operation, the equipment is subjected to a harsh thermal shock which can easily damage the filters (e.g. typically ceramic type filters, such as candle filters), and the turbine. As in normal operation (normal shutdown procedures), as a result of the operational disturbances care must be taken to ensure that the pressure differential between the reactor and the gas volume does not exceed a predetermined value, otherwise the walls of the reactor can collapse.
According to the present invention, a method and apparatus are provided which overcome the problems discussed above in operating a pressurized fluidized bed reactor, enabling safe operation both during normal operation, and as a result of operational disturbances. The invention is capable of effecting an almost immediate shutdown of the plant in such a way that damage does not occur to the components of the plant. Care is particularly taken to shut off the passage of relatively cool air to the reactor. The reactor is normally operated at high temperature, typically greater than 800.degree. C. (typically 800.degree.-1200.degree. C.), and if cold air flows substantially uncontrolled into the reactor rapid cooling of the hot components within and associated with the reactor would cause highly adverse thermal stress in the steel, refractory, and other components. Since the pressurized gas volume between the pressure vessel and the reactor is large, if the entire volume is allowed to vent into the reactor upon operational disturbance, damage to at least some of the reactor components, or the downstream filters, is almost certain to occur.
In addition to providing substantial advantages during operational disturbances, according to the present invention it is also possible to advantageously operate the reactor in a different manner even during normal operation. According to this aspect of the invention, not only is it possible to prevent collapse of the reactor due to too high of a pressure differential between the gas volume and the reactor, during normal operation it is possible to control the pressure differential by continuously controlling the passage of gas from the gas volume to the reactor, the gas being supplied as primary gas to the fluidized bed reactor.
According to one aspect of the present invention a method of operating a pressurized fluidized reactor power plant is provided. The power plant includes: a fluidized bed reactor supplied with fuel and contained within a pressure vessel with a pressurized gas volume defined between the reactor and the pressure vessel, a compressor for supplying gas under superatmospheric pressure to the gas volume, a first conduit for supplying primary gas from the gas volume to the reactor, a hot gas discharge from the reactor which passes through the pressure vessel, and a turbine operatively connected to the hot gas discharge. The method comprises the steps of, in response to an operational disturbance of the power plant: (a) Automatically terminating the passage of gas through the first conduit from the gas volume to the reactor. And, (b) generally simultaneous with step (a), automatically terminating the supply of compressed gas from the compressor to the gas volume. There is also preferably the further step (c) of generally simultaneously with steps (a) and (b), automatically terminating the flow of hot gas from the hot gas discharge to the turbine. There may also be the further step (d) of generally simultaneously with or after steps (a)-(c), substantially simultaneously automatically reducing the pressure in the gas volume and in the reactor. Also there may be the further step (e) of automatically monitoring the pressure differential between the rector and the gas volume, and practicing step (d) in response to step (e) so that the pressure differential between the reactor and gas volume does not exceed a predetermined amount. [The term "generally simultaneously" as used herein means simultaneously or somewhat before or after.]
Typically, but not necessarily, the first conduit extends from the gas volume to the outside of the pressure vessel, and then into the reactor, and a first valve is provided in the conduit substantially immediately adjacent (as close to as practical under the particular circumstances) the exterior of the pressure vessel. Then step (a) is practiced by substantially immediately automatically closing the first valve at the onset of the operational disturbance, so that from the onset of the operational disturbance a minimum volume of gas passes from the gas volume to the reactor. Alternatively the valve may be disposed within the pressure vessel if desired.
The compressor is also outside the pressure vessel and connected to the gas volume by a second conduit having a second valve therein substantially immediately adjacent and exteriorly of the pressure vessel. In that case step (b) is practiced by substantially immediately automatically closing the second valve at the onset of the operational disturbance, so that from the onset of the operational disturbance a minimum volume of gas passes from the compressor to the gas volume. There may also be the further step of, generally simultaneously with step (b), directing compressed gas directly from the compressor to the turbine.
According to another aspect of the present invention a method of operating a pressurized fluidized bed power plant is provided which comprises the following steps: (a) Supplying gas under superatmospheric pressure from the compressor to the gas volume. (b) Combusting or gasifying fuel in the reactor, producing hot gases. (c) Passing the hot gases from the reactor to the turbine. And, (d) withdrawing gas under superatmospheric pressure from the gas volume, passing the gas out of the pressure vessel, and reintroducing the gas through the pressure vessel into the reactor at a substantially continuously controlled rate.
In the case of gasification, after step (b) there is also a post-combustion of the hot gases prior to introduction to the turbine. Step (d) is typically practiced to supply the gas as primary gas to the fluidized bed reactor, and secondary and tertiary gas can also be provided from the compressor. During an operational disturbance the supply of gases to the reactor and the gas volume can be terminated as described above.
According to another aspect of the present invention a pressurized fluidized bed power plant is provided which comprises the following components: A pressure vessel. A fluidized bed reactor contained within the pressure vessel with a pressurized gas volume defined between said reactor and the pressure vessel. A first conduit for supplying primary gas from the gas volume to the reactor. A compressor for supplying gas under superatmospheric pressure to the gas volume through a second conduit. A hot gas discharge from the reactor, including a portion which passes through the pressure vessel. Means for separating particles from the hot gas discharge. A turbine operatively connected to the hot gas discharge. A first automatically controlled valve disposed in the first conduit for allowing or preventing the supply of primary gas from the gas volume to the reactor. And, a second automatically controlled valve disposed in the second conduit for allowing or preventing the passage of gas at superatmospheric pressure from the compressor to the gas volume. The first and second valves are preferably disposed exteriorly of the pressure vessel and substantially immediately adjacent to it, so as to minimize the volume of gas which passes to the reactor or the gas volume after closing of the first and second valves due to an operational disturbance.
The power plant also preferably comprises a third conduit branching from the second conduit for supplying secondary and tertiary air through the pressure vessel into the reactor, with an automatically controlled valve disposed in each of the secondary and tertiary lines and also controlled by the controller.
Hot gases discharged from the reactor preferably pass through a ceramic filter arrangement or the like, before passing to the turbine. A pressure relief valve can be disposed in the hot gas discharge and in the first conduit, with the controller also controlling operation of the pressure relief valves. As disclosed in patent 4,744,212, valves can also be disposed in the second conduit and the hot gas discharge immediately adjacent the compressor and turbine, respectively, with a bypass valve for directing compressed air directly from the compressor to the turbine, and all the valves controlled by the controller. A pressure monitoring means also may be provided for monitoring the differential pressure between the reactor and the gas volume, and for providing this information to the controller, which then can control the various valves to ensure that pressure differential does not exceed a predetermined amount (which would cause damage to the reactor, or downstream components).
It is the primary object of the present invention to provide a method and apparatus for preventing damage to a pressurized fluidized bed reactor power plant components as a result of an operational disturbance, and to improve normal operation thereof. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.